In 3GPP (third generation partnership project), there has been a substantial activity related to heterogeneous networks, so called hetnets, during the past year. A hetnet deployment may be characterized in that such a system includes different access points with different downlink output power levels. Further, the heterogeneous network consists of a combination of various access points, such as macro eNBs, micro/pico eNBs, relays, femto/home eNBs, etc.
Micro/pico eNBs (or base stations) are typically assumed to have the same functionality as a macro eNB, but the micro/pico eNB uses a lower output power than the macro eNB. Relays can also be low power nodes, but they are mainly characterized by the fact that they are not directly connected to a wired backhaul, instead the relays transmit/receive data via a “donor eNB”.
Femto/home eNBs are typically low power nodes for which it is possible to apply a concept of Closed Subscriber Group (CSG). If CSG is activated, only users included in the CSG are allowed to connect to such a femto/home eNB. Other users will be barred from that eNB.
Downlink power imbalance, together with an RSRP (reference symbol/signal received power) based cell selection, creates a situation where the path loss to a low power cell (for example a micro cell) is much lower than towards a high power cell (for example a macro cell). Nevertheless, a communication device (or a UE) connects to the high power cell (or eNB) since the received signal is stronger because of higher output power from the high power eNB than from the low power eNB (femto/home eNB).
Uplink power control in LTE is typically path loss based, which may lead to that users (i.e. communication devices) connected to a low power eNB suffer intolerably in the Uplink from high power eNB users' (communication devices') interference, which high power users (communication devices) are situated close to the low power eNB. In FIG. 1, there is shown an example, in which a low power eNB is represented by a micro cell and a high power eNB by a macro eNB.
From a system perspective, the interference problem is particularly severe on the uplink control channel, i.e. the PUCCH, since these signals are not re-transmitted. Further, ICIC schemes are traditionally designed with data channels in mind. A dysfunctional control channel leads to increased outage probability (coverage problems), degraded downlink and uplink throughput in the low power cells etc.
An even more challenging scenario is when the low power eNB (for example a pico eNB) is only available for CSG users. In that situation, the non-CSG user (communication device) creating interference towards the low power cell might be located just next to the access point.